1. Field of the Invention
The present invention relates to a recording apparatus for recording an image onto a recording medium on the basis of data supplied from the outside.
2. Related Background Art
Generally, a power switch attached to the main body of an image recording apparatus is a switch to control a power supply circuit. When a power source of the image recording apparatus is ON, an electric power is supplied to a control circuit of the apparatus main body. When the power source is OFF, no electric power is supplied. However, there is an image recording apparatus such that when the power source is turned off, a special power off sequence is executed to deliver a paper or to protect a recording head, or in order to perform the protection and management by a timer circuit when the apparatus is not used (in the non-recording mode), the apparatus is controlled to a mode to execute the printing operation while supplying the power source to the control circuit or a mode not to execute the printing operation (hereinafter, such a non-printing mode is called a power off),
In the specifications based on, for instance, the centronics interface standard which is most general as a printer interface specification, a data bus is connected between a data supply apparatus and an image recording apparatus and signal lines to transmit and receive the following signals and other various kinds of signals are also connected: namely, a strobe signal (latch signal of data which is generated from the data supply apparatus: this signal will be referred to as an inversion STB hereinafter); an acknowledge signal (signal to indicate that the recording apparatus can receive the next data when this signal is set to the low "L" level: this signal will be referred to as an inversion ACK hereinlater); a busy signal (signal indicating that data can be received when this signal is set to the L level: this signal will be referred to as a BUSY hereinlater); an error signal (signal indicating that the image recording apparatus is in an error state when this signal is at the L level: this signal will be referred to as an inversion ERROR hereinlater); and other various kinds of signals. The handshaking is executed among apparatuses by using the above signals and data can be accurately received.
However, in the case where the on/off operations of the power source of the image recording apparatus are controlled to a mode to execute the printing operation and a mode not to perform the printing operation instead of the on/off modes of the power supply circuit, an I/F (interface) signal is also effective even when the power source of the image recording apparatus is OFF and such a state is substantially similar to the ordinary off-line state. The data supply apparatus connected to the outside cannot discriminate whether the image recording apparatus is in the power ON state or the power OFF state.
In the data supply apparatus, in many cases, various kinds of control lines are used for the controls which the signal lines inherently mean and are also used for a purpose to monitor the power source state of the image recording apparatus.
In such an image recording apparatus, in order to inform the on/off state of the power source to the data supply apparatus connected to the outside, there is also a case where under the above power OFF state, each signal output line is set into a high impedance by the means for limiting a power source of an interface circuit, and when the apparatus is set into the pull-up state, the signal output line is set to the high "H" level, and when it is set into the pull-down state, the signal output line is set to the low "L" level, thereby preventing that the data supply apparatus side transmits the unnecessary data to the image recording apparatus,
There is also an apparatus using a method whereby in addition to the use of those existing I/F signals, the signal indicative of the on/off of the power source of the image recording apparatus is additionally prepared for the special data supply apparatus and can be similarly detected.
However, even in case of using a method whereby the power supply of the interface circuit is limited and each line is held to a high impedance state or the like under the power off state of the image recording apparatus, a desired result as expected cannot always be obtained because of the following reasons,
A pull-up resistor is connected to each signal line for interface of the image recording apparatus for the purposes of the protection of the input, the level matching between C-MOS and TTL, the countermeasure for delay times, the use of an open collector circuit, and the like, respectively. In addition, an input/ output gate circuit is also connected to the line without being supplied with an electric power. Consequently, the circuit is likely to be unstable in dependence on their connecting states,
For example, an electric power of a certain degree is supplied to a certain signal line through the pull-up resistor of the data supply apparatus connected to the outside. Such a power component rises the potentials of the other lines through the pull-up resistor on the image recording apparatus side. There occurs a problem such that as the potential reaches a threshold value, the circuit operation becomes unstable.
Those interface signals are set on the assumption that both of the data supply apparatus and the image recording apparatus which are connected operate in the normal state as a prerequisite. No consideration is taken with respect to the case where various signal lines do not execute the inherent operations.
In the case where the signal line enters an unstable state, the data supply apparatus waits for the execution of the handshaking between the data supply apparatus and the image recording apparatus, so that there occurs a problem such that the data supply apparatus stops in a standby state and is not opened or receives wrong information or the like.
On the contrary, there is also considered a situation such that when the power source on the data supply apparatus side is OFF, an adverse influence is given by the electric power which is supplied through the pull-up resistor on the image recording apparatus side.